The Glossopharyngeal and Vagus Nerves

The glossopharyngeal (CN IX) and vagus (CN X) nerves are intimately related and similar in function. Both have motor and autonomic branches with nuclei of origin in the medulla. Both conduct general somatic afferents as well as general visceral afferents fibers to related or identical fiber tracts and nuclei in the brainstem and both have a parasympathetic, or general visceral efferent and a branchiomotor, or special visceral efferent component. The two nerves leave the skull together, remain close in their course through the neck, and supply some of the same structures. They are often involved in the same disease processes, and involvement of one may be difficult to differentiate from involvement of the other. For these reasons the two nerves are discussed together.

THE GLOSSOPHARYNGEAL NERVE

Cranial nerve IX is difficult to examine because most or all of its functions are shared by other nerves and because many of the structures it supplies are inaccessible. It is possible to examine pain and touch sensation of the pharynx, tonsilar region, and soft palate, and the gag reflex. The only muscle to receive its motor innervation purely from CN IX is the stylopharyngeus. The only deficit that might be detectable is a slight lowering of the palatal arch at rest on the involved side. Other palatal motor functions are subserved either by CN X, or the two nerves working together.

The gag reflex is elicited by touching the lateral oropharynx in the region of the anterior faucial pillar, or by touching one side of the soft palate or uvula, with a tongue blade, applicator stick, or similar object. The afferent limb of the reflex is mediated by CN IX and the efferent limb through CNs IX and X. The reflex center is in the medulla. The motor response is constriction and elevation of the oropharynx. This causes the midline raphe of the palate and the uvula to elevate, and the pharyngeal constrictors to contract. The activity on the two sides is compared. The gag reflex is protective; it is designed to prevent noxious substances or foreign objects from going beyond the oral cavity. There are three motor components: elevation of the soft palate to seal off the nasopharynx, closure of the glottis to protect the airway, and constriction of the pharynx to prevent entry of the substance.

When unilateral pharyngeal weakness is present, the midline raphe will deviate away from the weak side and toward the normal side. This movement is usually dramatic. Minor movements of the uvula and trivial deviations of the raphe are not of clinical significance. There is variation in the intensity of the stimulus required. The gag reflex may be bilaterally absent in some normal individuals. Unilateral absence signifies a lower motor neuron lesion. Like most bulbar muscles
the pharynx receives bilateral supranuclear innervation, and a unilateral cerebral lesion does not cause detectable weakness.

The gag reflex is often used to predict whether or not a patient will be able to swallow or guard the airway. A decreased gag reflex may portend inadequate guarding of the airway and increased aspiration risk, but the status of the gag reflex is not a completely reliable indicator. Patients with an apparently intact gag reflex may still aspirate, and a patient with a depressed gag reflex may not. The clinical assessment of swallowing, a major component of which is the status of the gag reflex, underestimates the probability of aspiration in patients who are at risk, and overestimates it in patients who are not. The gag reflex may be hyperactive in some normal individuals, even to the point of causing retching and vomiting. A hyperactive gag reflex may occur with bilateral cerebral lesions, as in pseudobulbar palsy and amyotrophic lateral sclerosis (ALS).

Disorders of Function

Isolated lesions of CN IX are extremely rare if they ever occur. In all instances, the nerve is involved along with other cranial nerves, especially CN X. In glossopharyngeal neuralgia or “tic douloureux of the ninth nerve,” the patient experiences attacks of severe lancinating pain originating in one side of the throat or tonsilar region and radiating along the course of the eustachian tube to the tympanic membrane, external auditory canal, behind the angle of the jaw and adjacent portion of the ear. There may be trigger zones, usually in the pharyngeal wall, fauces, tonsilar regions, or base of the tongue. The pain may be brought on by talking, eating, swallowing, or coughing. It can lead to syncope, convulsions, and rarely to cardiac arrest because of stimulation of the carotid sinus reflex. Glossopharyngeal neuralgia must be differentiated from other craniofacial neuralgias, and from pain due to a structural lesion of the nerve.

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